Thrombotic complications are a major cause of death in the industrialized world. Examples of these complications include acute myocardial infarction, unstable angina, chronic stable angina, transient ischemic attacks, strokes, peripheral vascular disease, preeclampsia/eclampsia, deep venous thrombosis, embolism, disseminated intravascular coagulation and thrombotic cytopenic purpura. Thrombotic and restenotic complications also occur following invasive procedures, e.g., angioplasty, carotid endarterectomy, post CABG (coronary artery bypass graft) surgery, vascular graft surgery, stent placements and insertion of endovascular devices and protheses. It is generally thought that platelet aggregates play a critical role in these events. Blood platelets, which normally circulate freely in the vasculature, become activated and aggregate to form a thrombus with disturbed blood flow caused by ruptured atherosclerotic lesions or by invasive treatments such as angioplasty, resulting in vascular occlusion. Platelet activation can be initiated by a variety of agents, e.g., exposed subendothelial matrix molecules such as collagen, or by thrombin which is formed in the coagulation cascade.
An important mediator of platelet activation and aggregation is ADP (adenosine 5′-diphosphate) which is released from blood platelets in the vasculature upon activation by various agents, such as collagen and thrombin, and from damaged blood cells, endothelium or tissues. Activation by ADP results in the recruitment of more platelets and stabilization of existing platelet aggregates. Platelet ADP receptors mediating aggregation are activated by ADP and some of its derivatives and antagonized by ATP (adenosine 5′-triphosphate) and some of its derivatives (Mills, D. C. B. (1996) Thromb. Hemost. 76:835–856). Therefore, platelet ADP receptors are members of the family of P2 receptors activated by purine and/or pyrimidine nucleotides (King, B. F., Townsend-Nicholson, A. & Burnstock, G. (1998) Trends Pharmacol. Sci. 19:506–514).
Recent pharmacological data using selective antagonists suggests that ADP-dependent platelet aggregation requires activation of at least two ADP receptors (Kunapuli, S. P. (1998), Trends Pharmacol. Sci. 19:391–394; Kunapuli, S. P. & Daniel, J. L. (1998) Biochem. J. 336:513–523; Jantzen, H. M. et al. (1999) Thromb. Hemost. 81:111–117). One receptor appears to be identical to the cloned P2Y1 receptor, mediates phospholipase C activation and intracellular calcium mobilization and is required for platelet shape change. The second platelet ADP receptor important for aggregation mediates inhibition of adenylyl cyclase. Molecular cloning of the gene or cDNA for this receptor (P2Y12) has recently been reported (Hollopeter, G. et. al. (2001) Nature 409:202–207). Based on its pharmacological and signaling properties this receptor has been previously termed P2YADP (Fredholm, B. B. et al. (1997) TIPS 18:79–82), P2TAC (Kunapuli, S. P. (1998), Trends Pharmacol. Sci. 19:391–394) or P2Ycyc (Hechler, B. et al. (1998) Blood 92, 152–159).
Various directly or indirectly acting synthetic inhibitors of ADP-dependent platelet aggregation with antithrombotic activity have been reported. The orally active antithrombotic thienopyridines ticlopidine and clopidogrel inhibit ADP-induced platelet aggregation, binding of radiolabeled ADP receptor agonist 2-methylthioadenosine 5′-diphosphate to platelets, and other ADP-dependent events indirectly, probably via formation of an unstable and irreversible acting metabolite (Quinn, M. J. & Fitzgerald, D. J. (1999) Circulation 100:1667–1667). Some purine derivatives of the endogenous antagonist ATP, e.g., AR-C (formerly FPL or ARL) 67085MX and AR-C69931MX, are selective platelet ADP receptor antagonists which inhibit ADP-dependent platelet aggregation and are effective in animal thrombosis models (Humphries et al. (1995), Trends Pharmacol. Sci. 16, 179; Ingall, A. H. et al. (1999) J. Med. Chem. 42, 213–230). Novel triazolo[4,5-d]pyrimidine compounds have been disclosed as P2T-antagonists (WO 99/05144). Tricyclic compounds as platelet ADP receptor inhibitors have also been disclosed in WO 99/36425. Piperazine derivatives are described in WO 02/098856. The target of these antithrombotic compounds appears to be the platelet ADP receptor mediating inhibition of adenylyl cyclase or P2Y12.
Despite these compounds, there exists a need for more effective platelet ADP receptor inhibitors. In particular, there is a need for platelet ADP receptor inhibitors having antithrombotic activity that are useful in the prevention and/or treatment of cardiovascular diseases, particularly those related to thrombosis.